The present invention relates to a solvent-based adhesive composition for permanent bonding of synthetic rubber sheeting, and more particularly, to a solvent-based adhesive composition which exhibits excellent adhesion, peel strength, static load strength, and storage stability, and is curable on a job site.
Roofing materials for covering large roof areas are customarily prepared in wide sheets for installation. Elastomeric ethylene-propylene-nonconjugated diene terpolymer (EPDM) and isobutylene-conjugated diene copolymer (butyl rubber) compositions are well known in the art as roofing materials due to their barrier properties against moisture. These sheets must be overlapped and spliced together to form a continuous, water-tight sheet which covers a roof.
A common method of sealing the overlapping sheets together has been the use of adhesives such as solvent-based neoprene adhesives. Problems have been encountered with the bond strength of neoprene adhesives, and with the long term durability of those bonds due to moisture sensitivity.
The other commonly used solvent type adhesive is butyl rubber-based adhesives containing isocyanates as a portion of the curative components. However, these adhesives have limited shelf life and high toxicity.
Many efforts have been made to improve upon such solvent-based adhesives. For example, Nussbaum et al, U.S. Pat. No. 4,881,996, relate to a solvent-based adhesive composition comprising a halogenated pre-crosslinked butyl rubber, a thermoplastic copolymer, a hydrocarbon resin, a quinoid cure component mixture, and a solvent. However, while this composition avoids the necessity of a primer, it is believed to have a limited shelf life due to the very active curative components and a relatively low static load strength.
Chmiel et al, U.S. Pat. No. 4,603,164, disclose an adhesive composition comprising a blend of a halogenated butyl rubber, a pre-crosslinked butyl rubber, a methylene-propylene-nonconjugated diene terpolymer, a petroleum hydrocarbon based aliphatic thermoplastic resin, and an aliphatic isocyanate. However, while this composition exhibits good adhesion characteristics, it is believed to have limited storage stability as exposure to moisture leads to gelation of the composition. Further, extra care must be taken when handling the composition due to the toxicity of the isocyanate component.
Close, U.S. Pat. No. 4,742,119, also teaches a solvent-based roofing adhesive composition which is cured in situ after application to a roofing membrane. Such prior art in situ cure adhesives, however, have typically exhibited low initial peel strengths. In addition, use of such solvent-based adhesives may allow sliding of the seam in the shear direction to occur before there is enough time for the adhesive to cure and gain sufficient strength to hold the rubber sheets, and thus the seam, intact.
Additionally, other attempts have been made in the art to develop adhesive tapes which may be applied more readily and which provide a long term capability to withstand moisture penetration. For example, Chiu, U.S. Pat. Nos. 4,588,637, 4,855,172, and 5,095,068, teaches a roofing adhesive tape of a cured butyl rubber-based composition made by compounding a butyl rubber copolymer, a curing agent for the butyl rubber, carbon black, and a compatible tackifier. However, such adhesive tapes require additional fabrication steps to pre-cure the composition and then form it into a tape prior to its application.
Accordingly, the need exists in the art for a solvent-based adhesive composition for adhering together sheets of EPDM-based roofing materials which provides ease of application and curing on the job site. There is also a need in the art for such a solvent-based adhesive which provides improved levels of adhesion, peel strength, and static load strength, as well as possessing storage stability.